This invention relates to an air conditioning system for automotive vehicles, and more particularly to a system of this kind which is provided with a variable capacity compressor in which the delivery quantity or capacity is varied by utilizing the difference between pressure in a high pressure zone and pressure in a low pressure zone, in response to a thermal load on the system, etc.
Air conditioning systems are known, which have a variable capacity compressor constituting a refrigerating circuit together with an evaporator, etc., wherein the delivery quantity or capacity of the compressor is varied by bringing a high pressure zone into and out of communication with a low pressure zone, by means of a control valve having a bellows expansible and contractible in response to suction pressure of the compressor.
The conventional basic concept of the above method of controlling the delivery quantity of the compressor by utilizing suction pressure which is representative of a thermal load on the air conditioning system and the rotational speed of the compressor, that is, a so-called internally controlling method, is as follows:
An air conditioning system in general is required to satisfy the requirements that the range of discharge air temperature that can be controlled should be limited to such a range within which the evaporator will not freeze, and the discharge air temperature should be maintained at a constant level set within the above range during operation of the system. To fulfill the latter requirement, the evaporating pressure of the evaporator should be maintained at a constant level corresponding to the set level of the discharge air temperature. In the aforesaid known air conditioning systems, suction pressure in the compressor is directly introduced into a chamber formed therein and accommodating the bellows, and the delivery quantity is controlled by contraction and expansion of the bellows in response to the suction pressure so as to maintain the suction pressure constant and thereby maintain the evaporating pressure at the constant level. However, in actuality, if the evaporating pressure is kept constant, the suction pressure is varied along the line shown in FIG. 1, which is caused by a loss in pressure within a hose (low pressure hose) connecting between the suction port of the compressor and the outlet of the evaporator due to flow resistance of the hose arranged therebetween. The pressure loss can vary in dependence upon the diameter, length, bending manner of the hose, etc. This makes it difficult to control the evaporating pressure and hence the discharge air temperature at a constant level by means of the suction pressure, which will cause freeze-up of the evaporator or increase in the discharge air temperature, resulting in poor cooling performance of the system.
Further, there are variations in the loss in pressure between different types of vehicles. For example, an F. F (front engine and front drive) type vehicle has a relatively long low pressure hose which causes a great loss in the pressure, while an F. R (front engine and rear drive) vehicle has a relatively short low pressure hose which causes a small loss in the pressure. Thus, it is required to match the characteristics of the compressor with various types of vehicles. Especially, since a vehicle under developement has its design often changed, matching operation is needed every time the design is changed, which causes an increase in the manufacturing cost.